Focus on your audience. Think of the reader of your application--is this opportunity for something professional, something academic, or something artistic? Consider what he will be looking for in an applicant.
Determine what sets you apart from the pack. Think about the things that make you unique, whether it's where you grew up, the places you have been, jobs that you've had or other things you have accomplished.
Develop a theme. Your personal statement should be consistent in that you introduce an idea and follow through with examples, finishing by explaining why this makes you a valuable candidate. Perhaps the theme of your statement will be perseverance in the face of adversity, or volunteerism, or your personal relationship with the current opportunity.
Give examples. Don't simply write that you love working with underprivileged children--tell a story about the summer you spent building a kindergarten for impoverished students. By giving specific examples, you give a clearer impression of who you are and what you have done.
Explain why you are a good fit. The person to whom you are writing knows that her institution is a good fit for you--that's why you're applying. You need to explain what you have to offer the institution, and what you believe you can offer it in the way of accomplishments, work and attitude.
Tailor a unique personal statement for each institution to which you are applying. Do not write a generic personal statement to send to everyone--it should make specific reference to the institution in question directly in the body of the statement.
Ask trusted friends, family members and teachers to review each draft of your personal statement. If possible, ask people who have been in positions that required reading personal statements--they can help you strategize and suggest things to avoid.
Proofread and look for grammatical errors. Errors in your personal statement make you appear careless and unintelligent.